Traffic Collision Avoidance System (TCAS), independent of any ground inputs, performs surveillance of nearby aircraft to provide information on the position and altitude of these aircraft so collision avoidance algorithms can perform their function. TCAS surveillance operates by issuing interrogations at 1030 MHz that transponders on nearby aircraft respond to at 1090 MHz. These replies are received and decoded by a surveillance portion of TCAS software and the information is then provided to the collision avoidance algorithms.
TCAS has a requirement to provide reliable surveillance out to a range of 14 nautical miles (nmi) and in traffic densities of up to 0.3 aircraft per square nmi. The surveillance function provides the range, altitude, and bearing of nearby aircraft to the collision avoidance so threat determinations can be made and so the information displayed on the traffic display is accurate. TCAS can simultaneously track at least 30 transponder-equipped aircraft within its surveillance range.
The issue is that TCAS includes algorithms, which require it to limit the number and power of interrogations (see RTCA DO185A section 2.2.3.6.1). The limits imposed on TCAS are a function on the number of TCAS within approximately 30 nmi. The number of TCAS is detected via the UF=16 TCAS Broadcast transmission. This transmission contains the Mode S address of the TCAS aircraft and indicates that there is an operational TCAS on board the aircraft. The UF=16 TCAS Broadcast transmission is detected by Mode S transponders of other aircraft and passed on to the other aircraft's TCAS. As the number of TCAS equipped aircraft detected increases, TCAS power is decreased and receiver sensitivity is increased. In addition the algorithm attempts to determine if the distribution of TCAS within 6 nmi of own aircraft is linear in Range or linear in Area. If it is linear in range—TCAS transmit power and sensitivity can be further reduced—the assumption is that when TCAS are distributed linearly in range, the aircraft must be in close proximity to an airport and therefore—further limiting TCAS interrogation power and sensitivity is important so that TCAS does not interfere with ground radar.
However, if a military formation of several aircraft is TCAS equipped and spaced in such a manner to be distributed linearly in range from some other TCAS equipped aircraft, that other aircraft would reduce its transmission power to stay within the limits imposed by the number of TCAS in the area—and the linear range distribution.
The TCAS Broadcast transmission is not used for determining range—it is only used to communicate the Mode S address of a TCAS equipped aircraft to other TCAS equipped aircraft in the area—via the other aircraft's Mode S transponder.
Therefore, there exists a need for making sure that aircraft do not unnecessarily reduce their sensing power in the presence of aircraft flying in formation.